Everyone is talking about a new metabolism study published in August 2021.
There’s a reason why – this new research, looking at how aging impacts metabolism, is groundbreaking.
It shows us how little we know about metabolism, and how many things we thought we knew, were probably wrong.
For example, we have always thought that as we age, our metabolic rate goes down. One theory says that when we hit 35, our metabolism decreases by a few percentage points every year.
Also, common knowledge dictated that when women hit menopause, they often gain weight because of a metabolic dip due to estrogen loss. Or at least, that’s what we hypothesized (because it was never actually proven that lack of estrogen is the reason for menopausal weight gain).
But are these beliefs really true? Or has this latest metabolism study just smashed them to bits?
Let’s take a look.
What was the purpose of this study?
The study looked at the effects of age, body composition, and sex on energy expenditure, or metabolic rate, over the course of the lifetime.
Studies about metabolism are notoriously tough to do, because measuring metabolic rate, having a good range of ages, finding enough people to participate – these are all barriers.
This study overcame these barriers by collaboration. 80 scientists participated in the research study, pooling their evidence for 6421 subjects in 29 countries, from ages 8 days, to 95 years old.
It’s pretty much NEVER that a high-calibre study like this is published, so I’m very excited!!
I’m such a nerd.
What did the study find?
The study found that there are four points in our lifetimes when metabolism changes.
Here is one of the many graphs from the study, which illustrates what I’ve written below. BEE is ‘basal energy expenditure,’ which is essentially the calories we burn just being alive – breathing, heart beating, brain functioning. As you see, our BEE is super-high in our first years, then stabilizes until around age 60, when it slowly goes on a downward trajectory.
Stage 1: Birth to age 1. When a child is born, their metabolic rate actually mirrors that of their mother, until around 1 month of age. At that point, their metabolism accelerates, expending a larger amount of energy (likely due to the work of growth).
Stage 2: Age 1 to age 20. During this phase, metabolic rate continues to increase proportional to the amount of fat-free mass a person has. This means that males have a higher metabolic rate, but adjusting for body size and composition, sex made no difference in the rate of decline after age 20.
Stage 3: Age 20 to age 60. This is where the most interesting results lie. For most people, metabolic rate remained static through these 40 years. Even women who were pregnant and post-partum showed a stable metabolic rate through those years.
Stage 4: Age 60 to age 90+: This is the age range at which researchers found a decline in metabolism, by around 0.7% per year. This means that at age 90+, metabolism is around 26% less than at middle age.
Let me explain, since I know you probably have this question: don’t we all have different metabolic rates, depending on our musculature and body size? Yes, but the study didn’t measure metabolic rates between people.
It measured trajectory of the metabolic rate through the life cycle. To keep them from confounding the results, researchers adjusted for body size and fat-free mass, along with other factors.
And while around they found around 25% of people were outliers, on either side of the curve, most of us have the same ups and down through our lives, at around the same time.
The stage 3 findings are the most interesting to me, because they go against pretty much everything we ever thought about middle age and metabolic rate.
Menopausal women, long thought to experience a slowing metabolism due to the decline in estrogen levels, also showed a stable metabolism regardless of hormone levels.
As an aside, there was never conclusive evidence showing that estrogen levels were the reason why menopausal women gain weight. It has always been thought to be a part of it, but we didn’t have causal evidence to say that estrogen was THE reason.
So why do we tend to gain weight around menopause, if not for a slowing metabolism?
And why do we gain weight steadily after age 20?
Herman Pontzer, the principal investigator for this study and author of the book Burn, told me this:
This is another big surprise from the study: we can’t blame the added pounds and decreased vitality in middle age on a ‘slow metabolism’. Instead, it must be other changes through our lives (for example, testosterone peaks in men’s 20s and declines thereafter) that are affecting the way we feel. Gaining weight through middle age looks to be a problem of diet and overconsumption, not metabolism.
I’d probably add a more sedentary lifestyle to that mix, as well. In our middle years, we tend to get swallowed by life’s demands – work, kids, aging parents – and this may result in us being less active.
Why does the metabolic rate begin to drop at age 60? The study suggests that the brain and liver, two of our most energy-hungry organs, make up around 65% of our energy expenditure.
Researchers hypothesize that after age 60, these organs have lowered energy requirements, leading to a decrease in metabolic rate.
If metabolism decreases after age 60, you would think that we’d gain weight during that time, but the researchers didn’t find that, either.
Despite metabolic decline, weight did not increase in this age group. Both fat free and fat mass was reduced. What was their explanation for this? A hypothesis that our bodies try to regulate the amount of calories consumed, with the amount we burn.
Older people were burning fewer calories, and therefore, eating fewer calories.
What does this change?
As far as practical, on the ground nutrition practice, not much.
For most of us, metabolism doesn’t take a dive on our 35th birthday. Or, when we start menopause. In fact, it starts declining after age 60, and only by a fraction of a percent each year.
When I asked him how this study may change medicine, Pontzer told me this:
This is a new roadmap for energy expenditure across the lifespan. The elevated expenditures in early childhood through late adolescence reflect greater energy requirements for children, but also reflect something fundamental about development: our cells are much busier in childhood and adolescence than we’ve appreciated before, and that may have knock on implications for understanding and treating disease in children. Likewise, the decline in size-adjusted metabolism at 60+ suggests our cells are slowing down – a potential target for therapies to reduce or reverse aging.
This all shows how incredible our bodies are – to keep a delicate balance through much of life, including in pregnancy and in menopause.
Our bodies are so complicated, and this is why I always get after people who claim to ‘fix’ metabolisms. Researchers are just starting to figure this stuff out now. Stick with the science.